To fight the SARS-CoV2 pandemic, the European Medicines Agency (EMA) has approved the ChAdOx1 nCov-19 vaccine marketed by AstraZeneca. ChAdOx1 nCov-19 is based on chimpanzee adenovirus Y25 and encodes for the SARS-CoV2 spike protein. Following vaccination, rare cases of vaccine-induced immune thrombotic thrombocytopenia (VITT) have been reported. The initial trigger for these serious adverse events has not been determined. Initially intended to exclude any product-related issues, we analysed the ChAdOx1 nCov-19 vaccine by biochemical and proteomic methods. Unexpectedly, we found that the vaccine, in addition to the adenovirus vector, contained both non-structural viral proteins and substantial amounts of human host cell protein (HCPs). Protein amounts demonstrably exceeded the  specification limit per vaccine dose set by the EMA. Among the human proteins, heat-shock proteins and cytoskeletal proteins were particularly abundant. It is possible that the often-observed strong clinical reaction one or two days after vaccination might be associated with the detected protein contaminants. A linkage to later immune-related adverse events is also conceivable. In contrast, in lots of the second approved adenoviral vector-based SARS-CoV2 vaccine Ad26.COV2.S marked by Johnson&Johnson, only negligible amounts of HCPs were detected, confirming that excellent purification of clinical grade adenovirus vaccines is possible at an industrial scale.

The here reported identification of specific classes of protein impurities in the ChAdOx1 nCov-19 vaccine should guide and accelerate efforts to improve the purity of adenoviral vector-based vaccines and quality assessment methods to increase safety and efficacy.